Cylindrical dryers are used in the paper making process. Webs of paper are passed over heated cylindrical drums to remove moisture from the web. The large cylindrical dryers, often referred to as “Yankee Dryers,” must be continuously heated to maintain an elevated temperature during the paper making process.
One type of Yankee dryer has an inner and outer shell. The space created between the inner and outer shell is fed with a heating medium, such as steam under pressure, to heat the outer surface of the dryer. The dryers are commonly made out of cast iron. A double shelled cast iron dryer is difficult to cast, costly and extremely heavy. Double shelled dryers were very rare and the idea was abandoned early.
Another type of Yankee dryer has a closed cylinder with pressurized steam fed into the cylinder. The pressurized steam raises the possibility of catastrophic explosion when the cylinder fails under the pressure. One possible solution to explosion risks in a pressurized cylinder type Yankee dryer is to fill the volume within the cylinder with spheres. Spheres occupy space within the cylinder and reduces the amount of pressurized steam. This reduced amount of pressurized steam lowers the risk of explosions. Problems with this approach include the need to use a non-compressible material for the spheres, increasing the weight of the dryer. Also, with spheres of equal size the total volume cannot be reduced more than approximately two thirds. This reduction is not enough for the purposes of reducing the amount of steam.